The present invention is concerned with a method and apparatus for multiuser interference cancellation in a system comprising a radio base station serving several transmitting radio stations. The invention is also concerned with an interference cancellation unit for use in such systems.
Radio transmission of information is carried out by modulation of a carrier wave and transmitting the modulated signal with a radio transmitter. The receiver demodulates the signal to reproduce the information sent. The demodulation and modulation techniques depend on the multiple access method used.
Different multiple access methods exist for the transmitting and receiving of the radio signals. In TDMA, Time Division Multiple Access, a channel consists of a time slot in a periodic train of time intervals over the same frequency. In FDMA, Frequency Multiple Access, a communication channel is a single radio frequency band. Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. In Code Division Multiple Access, CDMA, the signals can share the same frequency band and the CDMA receiver can also operate with several frequency bands at the same time. The selection, i.e. discrimination, between the desired signal and other signals is carried out by suitable signal processing, which is based on a spreading code used to modify the desired signal. All simultaneous connections use different codes.
In the CDMA system, the radio signal is received by an antenna and filtered so that signals of other frequencies would not interfere. The result is demodulated to a bandlimited base band signal that can be fed to a base band processing unit to reproduce the transmitted digital data stream, which had been modulated by for example spread spectrum technique. In spread spectrum, radio signals are transmitted by expanding the bandwidth of the information signal by means of an independent code signal. The band spreading is achieved so that each fed information bit is replaced by a code sequence. If the receiver is authorized and has a syncronous code signal then the corresponding information signal can be despread and demodulated. One CDMA technique uses a signature sequence to represent one bit of information. The signature sequence comprises M bits and each bit is called a chip. The entire M-chip sequence is referred to as a transmitted symbol. The despreading code isolates the signals of the desired station and reduces the signals of other stations to noise.
Interference from the other transmitting radio stations served by the base station occur in all kind of radio receiver systems, such as CDMA, TDMA and FDMA systems mentioned above, as the antenna of the radio receiver system receives a radio signal, which is a combination of signals from some or all the transmitting radio stations (or mobile stations) in a cell or sector of a cell.
Even interference from mobile stations from other base stations might occur. There are methods, with which interference from such mobile station can be cancelled without knowledge about the transmitting mobile station. Such interference might be received by an antenna and demodulated, even though the detector will ignore them in a later stage.
In CDMA, it means that the demodulator of a CDMA receiver system produces a base band signal which is the sum of the base band signals from some or all of the transmitting radio stations. The number of radio stations that can share the same frequency band in CDMA is therefore limited by co-channel interference. Thus, there are several users on the same frequency band using different spreading codes which might interfer with each other
In a cellular communications system, for example, such interference limits the number of mobile stations that can access the same base station. The communication quality decreases with an increased number of mobile stations. It is therefore preferable to take information of other interferring signals into consideration at the receiver in decoding the received signal to cancel co-channel interference in the radio receivers.
There are many different interference cancellation (IC) methods. In some of them the interference from other transmitting radio stations is calculated based on an estimate of its transmitted symbol, and the estimated interference is deducted from the total received signal. The interference might also be calculated on the basis of spreading sequences and time delays, for example using a decorrelating detector. All users"" interference signals may be estimated in parallell, in which the estimates are refined over a number of stages; or in serial, where the user signals are ranked according to their reliability; or a hybrid combination of the serial and parallell methods. A description of parallell and serial interference cancellation methods can be found in xe2x80x9cPERFORMANCE OF AN ADAPTIVE SUCCESSIVE SERIALxe2x80x94PARALLELL CANCELLATION SCHEME IN FLAT RAYLEIGH FADING CHANNELS, Tik-Bin Oon, Raymond Steele and Ying Li, Department of Electronics and Computer Science, Univ. of Southampton, SOI17 1BJ, UK.
In the interference cancellation method of U.S. Pat. No. 5,579,304, the information of other users"" signals is made use of so that one of the user signals of a base station is detected and the bandlimited signal is demodulated to reproduce the original. information signal. The signal thus produced is subtracted from the combined signal and the result signal gives information of the rest of the user signals. The following user signal is detected in a similar way and the corresponding subtraction is made from the result signal from the foregoing step. A new signal is produced in a similar way after processing of each user signal and the whole process can be repeated several times.
In the above methods, each interference-canceling stage generates estimated symbol values for the mobile stations, later stages refining the estimates of earlier stages. Each time a symbol value is thus estimated or re-estimated, corresponding information is removed from the base band signal.
If there are several users in the same base station, the method becomes very complex and difficult to implement. The number of necessary calculations for each of these methods is quite large and increases as the number of the serving base station""s subscribers increases. Receiving of signals from other subscibers might also influence and should be taken into consideration.
In WCDMA (Wide band CDMA) systems, a large number of mobile users are served by one base station. The base station needs to process and receive all these users"" signals; a task which is processing power consuming and very difficult to accomplish; The introduction of an interference cancellation unit to the system further complicates the task since now a significantly larger number of user signals would need to be processed at the receiver.
The object of the invention is to make the processing of interferring signals less complex in systems with a large number of mobile users.
It is therefore suggested, according to the present invention, to process the user signals separately in at least two operations.
The multiuser interference cancellation method of the invention is used in a radio receiver which receives signals within the range of the receiver in a system with several transmitting radio stations. Interference from other transmitting radio stations is cancelled so that a desired signal cleaned from interfering signals would be obtained. The transmitting radio stations use a particular multiple access method for transmitting modulated radio signals. The radio receiver receives a radio signal as a sum of the signals from at least two transmitting radio stations in the system. The radio signal is processed in accordance with the multiple access method used, whereafter interference is cancelled from at least some of the transmitting radio stations in the system from the radio signal. The desired modulated signal sent to the receiver is then estimated. The method of the invention is mainly characterized in that the interfering signals from at least some of the transmitting radio stations are processed in at least two separate operations in one or more stages, the result of each separate operation being taken into consideration in the estimation of the desired signal.
Preferably, a part of the interfering signals, constituting a group of user signals, is processed in one of the separate operations, while the rest of the interfering signals, constituting another group or other groups of user signals, are processed in the other separate operation(s).
If the cancellation is carried out in more than one step, the cancellation results of the first cancellation stage are used in the second interference cancellation stage, the method being correspondingly continued in the possible further cancellation stages.
The apparatus and the multi-user interference cancellation unit of the invention are mainly characterized in that the multi-user interference cancellation unit consists of at least two physical units, over which the interfering signals are processed in separate operations in one or more stages.
The interference cancellation is preferably carried out over two or more physical units, one for each group of user signals, with one or more information transmissions between the units, so that all or part of the estimated interference signals are deducted from the total received signal. A physical unit may comprise one receiver board or a part thereof.
To carry out information transmissions between the units, the units comprise means for transferring information, between the units, of the signals from transmitting radio stations.
In one embodiment of the invention, the interference cancellation carried out separately for two or more groups of user signals in at least one stage in each physical unit is carried out to achieve estimated interference signals from each separate operation, whereafter the estimated interference signals are deducted from the total received signal as a sum of total of user signals cancelled in each separate operation.
This embodiment can be carried out so that the estimated signal values are used in the next cancellation stages to further improve these values and the information transmission between the units consists of cleaned signals that are sent to the other units to be taken into consideration in the next cancellation stages.
In an alternative the embodiment of the invention, the information transmission between the units consists of total estimated interfering signals.
In another embodiment of the invention, the transmission between the units is in form of an output signal from each physical unit, consisting of a composite data stream which contains information about the users which were assigned to the unit in form of detected values of each users received data, the associated channel estimates, time delay estimates, spreading codes and such information and is sent to the other units so they can generate estimated values or similar values for the users in the other units for cancelling the associated interference from their assigned users.
Different interference cancellation methods can be used. In some of them the interference from the other transmitting radio stations in the system is calculated on the basis of the estimate of their transmitted symbols. The interference might also be calculated on the basis of spreading sequences and time delays, for example using a decorrelating detector. The interference cancellation methods used in the separate interference cancellation operations can be serial interference cancellation methods, parallel interference cancellation methods, or hybrid methods of these and they can be carried out with the same method or with different methods in the different units.
In the invention, the amount of inter-board communications for interference cancellation purposes is kept low without significantly affecting the IC performance.
Hardware is used for implementation of the interference cancellation method of the invention, which allows the functions to be performed over two or more physical receiver units. Associated hardware is used at the base station for implementing the method.
The range of the receiver in a system with several transmitting radio stations within which radio signals are received is mainly the one served by the base station, but the reciever might also receive signals from mobile stations served by other base stations.
The method can be used in FDMA, TDMA and CDMA systems and is especially useful in all types of CDMA systems, including WCDMA. The invention has an other advantage in that a pool of receiver boards each containing one or more of said physical units, can exist at the base station and can be assigned to a particular sector as traffic loads vary within the cell.
The invention will now be described by means of the following figures, which presents examples of different advantageous embodiments of the invention. The invention is not restricted to the details of the text and figures, as the details of the invention can vary within the scope of the claims.